Our story begins, somewhat arbitrarily, in the English city of Manchester around the turn of the nineteenth century. There, a child prodigy by the name of John Dalton, at the tender age of fifteen is teaching in a school with his older brother. Within a few years, John Dalton’s interests have developed to encompass meteorology, physics, and chemistry. Among the questions that puzzle him is why the various component gases in the air such as oxygen, nitrogen, and carbon dioxide do not separate from each other. Why does the mixture of gases in the air remain as a homogeneous mixture? As a result of pursuing this question, Dalton develops what is to become modern atomic theory. The ultimate constituents of all substances, he supposes, are hard microscopic spheres or atoms that were first discussed by the ancient Greek philosophers and taken up again by modern scientists like Newton, Gassendi, and Boscovich. But Dalton goes a good deal further than all of these thinkers in establishing one all-important quantitative characteristic for each kind of atom, namely its weight. This he does by considering quantitative data on chemical experiments. For example, he finds that the ratio for the weight in which hydrogen and oxygen combine together is one to eight. Dalton assumes that water consists of one atom of each of these two elements. He takes a hydrogen atom to have a weight of 1 unit and therefore reasons that oxygen must have a weight of 8 units. Similarly, he deduces the weights for a number of other atoms and even molecules as we now call them. For the first time the elements acquire a quantitative property, by means of which they may be compared. This feature will eventually lead to an accurate classification of all the elements in the form of the periodic system, but this is yet to come. Before that can happen the notion of atoms provokes tremendous debates and disagreements among the experts of Dalton’s day.
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